The invention relates to a receiver for use in a system wherein a signal to be transmitted is multiplied with a code sequence characteristic of each connection, the receiver comprising means for estimating a channel, and one or more demodulator means, and means for combining signals received from the demodulator means.
The invention further relates to a method for generating spreading codes in a receiver, in which method a signal to be transmitted is multiplied with a code sequence characteristic of each connection, and in which receiver signal components transmitted with the desired code are sought from the received transmission, and the phases of the components are measured by correlating the received transmission with the code sequence generated in the receiver.
The receiver and the method according to the invention can be applied especially in a cellular system utilizing code division multiple access.
CDMA (Code Division Multiple Access) is a multiple access method, which is based on the spread spectrum technique and which has been applied recently in cellular radio systems, in addition to the earlier commercially implemented FDMA and TDMA methods. CDMA has several advantages over the earlier implemented methods, for example spectral efficiency and the simplicity of frequency planning.
In the CDMA method, the narrow-band data signal of the user is multiplied to a relatively wide band by a spreading code having a considerably broader band than the data signal. In known test systems, bandwidths such as 1.25 MHz, 2.5 MHz and 25 MHz have been used. In connection with multiplying, the data signal spreads to the entire band to be used. All users transmit by using the same frequency band simultaneously. A separate spreading code is used over each connection between a base station and a mobile station, and the signals of the users can be distinguished from one another in the receivers on the basis of the spreading code of each user.
A CDMA receiver comprises means, which can be implemented for example with correlators or matched filters, for synchronization with a desired signal, which is recognized on the basis of the spreading code. In the receiver, the data signal is restored to the original band by multiplying it again by the same spreading code as was used during the transmitting stage. Signals multiplied by some other spreading code do not correlate in an ideal case and are not restored to the narrow band. They appear thus as noise with respect to the desired signal. The spreading codes of the system are preferably selected in such a way that they are mutually orthogonal, i.e. they do not correlate with each other.
In a typical mobile phone environment, the signals between a base station and a mobile station propagate along several paths between the transmitter and the receiver. This multipath propagation is mainly due to the reflections of the signal from the surrounding surfaces. Signals which have propagated along different paths arrive at the receiver at different times due to their different transmission delays. CDMA differs from the conventional FDMA and TDMA in that the multipath propagation can be exploited in the reception of a signal. One way of realizing a CDMA receiver is to use, for example, a so-called rake receiver, which consists of one or more rake branches. Each branch is an independent receiver unit, the function of which is to compose and demodulate one received signal component. Each rake branch can be caused to synchronize with a signal component which has propagated along an individual path, and in a conventional CDMA receiver the signals of the receiver branches are combined advantageously, for example coherently, whereupon a signal of good quality is obtained. The signal components received by the receiver branches may be transmitted from one base station, or in the case of macrodiversity, from several base stations. The realization of a rake branch is described in greater detail in G. Cooper, C. McGillem: Modern Communications And Spread Spectrum (McGraw-Hill, New York, 1986, Chapter 12).
In mobile network applications, the use of long spreading codes provides several advantages. A sufficient length of the spreading code enables almost an unlimited number of different code sequences (by means of which the signals of different users are distinguished from one another), the easy application of cryptographic algorithms, and the use of the same long code at different phases in synchronous networks. In connection with using long codes, the magnitude of the delay spread is unlimited.
The use of long codes has been difficult so far, however, since there are a number of problems related to their use. The code search periods may become long whereupon the synchronization is slow. When long codes are used, the network should typically be synchronous. The receiver must also detect the signal from a partial correlation result, which does not produce an ideal result. In a rake receiver, there may occur problems in code search, measurement of impulse response, the activation of rake branches to receive different signal components, code tracking and synchronization of transmitter and receiver directions. An example of a system utilizing long codes and having the aforementioned problems is the IS-95 standard suggestion, which is incorporated herein by reference.